An electronically commutated electrical machine, for example a synchronous machine or an asynchronous machine, comprises a stator and a rotor. The stator is used to provide a motor magnetic field. The rotor comprises permanent magnets and/or linked rotor coils in order to provide an exciter magnetic field.
The stator comprises a plurality of stator coils that are allocated in each case to one or several phases of the electrical machine. In order to operate the electrical machine as a motor, the stator coils are energized in a suitable manner so that the motor magnetic field is generated. The interaction between the motor magnetic field and the exciter magnetic field produces a driving torque that acts on the rotor.
The individual stator coils are energized in accordance with a commutation method, wherein in order to generate a maximum driving torque it is necessary to advance the motor magnetic field in the direction of the exciter magnetic field, which motor magnetic field is generated by the stator coils. Since the rotor moves with respect to the stator coils, it is necessary, for the purpose of suitably energizing the stator coils, to know the position of the rotor (rotor position) and thus the direction of the exciter magnetic field in order to be able to generate the motor magnetic field with the corresponding angle of advance. A maximum driving torque can be achieved when advancing the motor magnetic field by 90° with respect to the exciter magnetic field (electrical rotor position; corresponds to the mechanical rotor position divided by the number of pole pairs of the rotor).
For this reason, an internal position sensor for detecting the rotor position is generally provided in electronically commutated electrical machines. The energizing patterns are then applied to the corresponding stator coils in dependence upon the rotor position that is detected by the position sensor. Generally, the internal position sensor is dimensioned such that it provides sufficient position resolution in order to perform the commutation.
When using an electronically commutated electrical machine for a positioner, it is possible in order to reduce the costs of detecting the rotor position to forego an internal position sensor and to detect only a position of the actuator that is operated by the electrical machine. When using a so-called external position sensor for the purpose of ascertaining the position of the actuator, both the cost for the electrical machine and also the cost for providing the cabling between the electrical machine and the control device are reduced and this constitutes an advantage. However, the actuator is frequently connected to the electrical machine by way of a reduction gear, so that it is not possible to obtain accurate information regarding the rotor position from the position of the actuator owing to the reduction ratio and owing to the unavoidable amount of play. It can be expedient for reasons of cost to limit the accuracy of the resolution of the position sensor even when using an internal position sensor.
The inaccuracies when ascertaining the rotor position from the position of the actuator result in it not being possible to adjust the angle of advance to the optimum angle of advance of 90°. Consequently, when using an external position sensor for the purpose of operating the electronically commutated electrical machine there is the disadvantage that it is not possible to operate the electrical machine at a maximum torque.
It is therefore the object of the present disclosure to provide an improved commutation of an electrically commutated electrical machine, where the information that is provided regarding a rotor position is less than accurate.